1. Field of the Invention
The present invention relates to a microwave plasma processing apparatus and a method of supplying microwaves, and more particularly, to a microwave plasma processing apparatus that performs plasma-processing on an object to be processed by exciting gas due to the electric field energy of microwaves emitted from a radial line slot antenna (RLSA), and a method of supplying microwaves using the microwave plasma processing apparatus.
2. Description of the Related Art
Microwave plasma is generated by introducing microwaves into a processing container in a depressurized state and by exciting gas due to the electric field energy of the introduced microwaves. In microwave plasma processing apparatuses, when the electron density of plasma is higher than a cut-off density, microwaves cannot be absorbed into plasma and thus, are propagated between a dielectric plate and plasma, and some of the microwaves are absorbed into the plasma and are used to sustain the plasma.
According to the principle of generating plasma, since microwave plasma has a higher electron density (Ne) and a lower electron temperature (Te) than plasma that is generated by a capacitively coupled microwave plasma processing apparatus or an inductively coupled plasma processing apparatus, a high-quality device can be manufactured at high rate and with little damage by performing plasma processing.
A microwave plasma processing apparatus using a radial line slot antenna (RLSA) has been proposed (i.e., see Japanese Laid-Open Patent Publication No. hei 9-63793). The RLSA has a structure in which a wavelength-shortening plate having a disk shape is placed on a disk-shaped slot plate having a plurality of slots formed therein, and is disposed on a dielectric window formed in an opening of a ceiling part of a processing container. The middle part of the RLSA is connected to a coaxial waveguide.
In the above-described structure, microwaves of 2.45 GHz, for example, outputted from a microwave source, are transmitted to the wavelength-shortening plate of the RLSA via the coaxial waveguide so as to radiate in a radial direction of the wavelength-shortening plate. As such, microwaves can be emitted from the plurality of slots formed in the slot plate and can be radiated into the processing container.
However, during a plasma process, the processing container is maintained at a high temperature of 200° C. and more, and as a result, an RLSA 905, a cooling jacket 210, an outer conductor 340 of the coaxial waveguide, a rectangular waveguide 305, which are shown in FIG. 8, are thermally expanded. Thus, during the plasma process, even though a circumferential part of the RLSA 905 is cooled by the cooling jacket 210, the temperature of the RLSA 905 increases about 150° C. to about 165° C. and the temperature of the cooling jacket 210 placed above the RLSA 905 increases about 80° C. to about 100° C. and the temperature of the external conductor 340 increases about 40° C. to about 60° C., and a temperature of 100° C. and more may be heated up even near the outer conductor 340 according to the plasma process.
Referring to FIG. 8, among these members, a wavelength-shortening plate 905a (see FIG. 8) of the RLSA 905 is formed of a dielectric material such as alumina (Al2O3). Meanwhile, the cooling jacket 210, the outer conductor 340, and the rectangular waveguide 305, which are placed above the RLSA 905, are formed of metal such as copper (Cu) or aluminum (Al). The linear expansion coefficient of alumina is 7.0×10−6(/° C.). The linear expansion coefficient of copper is 16.7×10−6(/° C.) and the linear expansion coefficient of aluminum is 23.5×10−6(/° C.), which are more than twice that of alumina. Thus, when a temperature increases, the RLSA 905, the cooling jacket 210, the outer conductor 340, and the rectangular waveguide 305 are thermally expanded, and thus an upper part of the rectangular waveguide 305 is displaced to a higher location than that before the temperature increases.
In this case, if a slot plate 905b of the RLSA 905 is screw-fixed to a taper-shaped connector part (hereinafter, referred to as a taper connector) attached to an inner conductor 315 of the coaxial waveguide, a coaxial converter 310, the inner conductor 315, and the taper connector, which are integrally formed as one body with one another, are displaced in a vertical upward direction of a processing container 100, following the outward displacement of the position of the rectangular waveguide 305 in an outside direction of the processing container 100.
In particular, the inner conductor 315 and the coaxial converter 310 allow a refrigerant to pass through to the outside of a refrigerant pipe 360 from the inside of the refrigerant pipe 360, which is a double pipe installed within the inner conductor 315, and thus are cooled even during the plasma process. Thus, the temperature of the inner conductor 315 and the temperature of the coaxial converter 310 during the process are lower than the temperature of the outer conductor 340 and the temperature of the rectangular waveguide 305. Thus, the thermal expansion rate of the coaxial converter 310 and the inner conductor 315 during the plasma process is lower than the thermal expansion rate of the outer conductor 340 and the rectangular waveguide 305. As such, in contrast with an ideal state shown in the upper drawing of FIG. 9, in the lower drawing of FIG. 9, when a temperature increases, a lower surface of a taper connector 320 connected to the inner conductor 315 is displaced in an upper direction away from a lower surface of the wavelength-shortening plate 905a, and an air gap Ra between the taper connector 320 and thus the wavelength-shortening plate 905a varies. The air gap Ra is part of a transmission path of the microwaves and thus, it is important to maintain the air gap Ra so as to stabilize a mode of the microwaves. If the air gap Ra varies, the mode of the microwaves is unstable, and plasma is non-uniform.
In addition, if, when a temperature increases, the taper connector 320 is displaced in an upper direction away from the wavelength-shortening plate 905a, the slot plate 905b screw-fixed to the lower surface of the taper connector 320 is also displaced upward and is distorted. Thus, the transmission path of the microwaves varies, and uniform plasma is not generated.